1. Field of the Invention
The present invention relates to an apparatus and method for adding distortion to a signal, to thereby compensate for distortion affecting the signal at a later stage by a nonlinear element.
2. Description of the Related Art
Nonlinear elements, such as amplifiers and laser circuits, have nonlinear characteristics which can produce nonlinear distortion. Devices are available which compensate for such nonlinear distortion.
For example, a predistorter system can be used to compensate for nonlinear distortion. Generally, with a predistorter system, a nonlinear circuit is positioned before the nonlinear element which produces the distortion. The nonlinear circuit has opposite characteristics than those of the nonlinear element, to thereby compensate for the distortion produced by the nonlinear element.
FIGS. 1(A), 1(B), 1(C), 1(D) and 1(E) are diagrams illustrating the operation of such a conventional predistorter system. Referring now to FIG. 1(A), a nonlinear element 1202 produces nonlinear distortion. A distortion compensating unit 1201 is positioned before a nonlinear element 1202, and has nonlinear characteristics opposed to that of nonlinear element 1202. An input signal is processed by distortion compensating unit 1201 and is then provided to nonlinear element 1202. Nonlinear element 1202 then produces a distortion compensated output signal. Alternatively, distortion compensating unit 1201 can be positioned after nonlinear element 1202.
FIG. 1(B) is a graph illustrating an input signal, FIG. 1(C) is a graph illustrating the input/output characteristics of distortion compensating unit 1201, FIG. 1(D) is a graph illustrating input/output characteristics of non-linear element 1202, and FIG. 1(E) is a graph illustrating an output signal. As can be seen from FIGS. 1(C) and 1(D), the nonlinear characteristics of distortion compensating unit 1201 are opposite the nonlinear characteristics of nonlinear element 1202. As a result, as can be seen from FIGS. 1(B) and 1(E), a distortion compensated output signal is produced.
FIG. 2 is a diagram illustrating a conventional distortion compensating unit 1201. Referring now to FIG. 2, an input signal is provided to a directional coupler 1301. Direction coupler 1301 branches the input signal into a first signal and a second signal. The first signal is provided to a delay element 1302, and the second signal is provided to a distortion component generating unit 1303. Distortion component generating unit 1303 generates a distortion component for the input signal. The distortion component is then adjusted in amplitude and phase by an attenuator 1304 and a phase adjustor 1305, respectively. The adjusted distortion component is provided to a directional coupler 1306.
On the other hand, the signal received by delay element 1302 is time delayed and provided to directional coupler 1306. The delay time corresponds to the processing time of distortion component generating unit 1303, attenuator 1304, and phase adjustor 1305. Directional coupler 1306 couples the time delayed signal with the distortion component.
However, the distortion compensating unit illustrated in FIG. 2 requires a phase shifting 90.degree. coupler and a 180.degree. coupler for canceling a primary signal element in distortion component generating unit 1303 and phase adjustor 1305. As a result, the distortion generating unit is relatively expensive to produce and is relatively large-scale. Moreover, a 90.degree. coupler can only be used for a limited frequency band. Therefore, the distortion compensating unit cannot be used to compensate for distortion over a broad band.
In addition, the distortion compensating unit illustrated in FIG. 2 can only be used to compensate for third order distortion since second order distortion becomes out of the band.
FIG. 3 is a diagram illustrating an additional, conventional distortion compensating unit 1201. Referring now to FIG. 3, an input signal f is received by a directional coupler 1401. Direction coupler 1401 branches the input signal into a first signal which is provided to an even-ordinal distortion generating unit 1423, and a second signal which is provided to an odd-ordinal distortion generating unit 1424.
The first signal is received by a 180.degree. coupler 1402 of even-ordinal distortion generating unit 1423. 180.degree. coupler 1402 branches the first signal into separate signals by shifting the phase of the first signal by 180.degree.. The two signals output from 180.degree. coupler 1402 are provided to diodes 1403 and 1407, respectively, and provided with distortion. A directional coupler 1411 combines the signals from diodes 1403 and 1407. Since the signals provided to diodes 1403 and 1407 are 180.degree. shifted in phase, an odd-ordinal distortion component generated by diodes 1403 and 1407 is canceled and an even-ordinal distortion signal (Af.sup.2 +Bf.sup.4 . . .) is output from directional coupler 1411. The Af.sup.2 and Bf.sup.4 correspond to the high frequencies of A sin2.omega.t and B sin4.omega.t, respectively.
The second signal branched by directional coupler 1401 is received by a directional coupler 1412 of odd-ordinal distortion generating unit 1424. Directional coupler 1412 branches the second signal into two separate signals which are provided to diodes 1413 and 1417, respectively, and provided with distortion. A directional coupler 1421 combines the signals from diodes 1413 and 1417. Diodes 1413 and 1417 are set in opposite directions. As a result, an even-ordinal distortion component generated by diodes 1413 and 1417 is canceled, and an odd-ordinal distortion signal (f +Cf.sup.3 . . .) is output from directional coupler 1421.
The even-ordinal distortion signal (Af.sup.2 +Bf.sup.4 . . .) generated by even-ordinal distortion generating unit 1423 and the odd-ordinal distortion signal (f+Cf.sup.3 . . .) generated by odd-ordinal distortion generating unit 1424 are combined by a directional coupler 1422, to thereby produce a distortion signal (f+Af.sup.2 +Cf.sup.3 +Bf.sup.4 . . .).
The distortion compensating unit illustrated in FIG. 3 can compensate distortion over a band of approximately DC - 500 MHz, and can adjust the odd- and even-ordinal distortions independently.
However, the distortion compensating unit illustrated in FIG. 3 is relatively costly and large-scale, since it requires the use of expensive, large scale directional couplers 1401, 1411, 1412, 1421, and 1422. Moreover, the distortion compensating unit is limited in frequency band by directional couplers 1401, 1411, 1412, 1421, and 1422. In addition, it is difficult to finely adjust distortion in such a distortion compensating unit.